During the manufacture of a semiconductor device a large number of transistors and other structures are formed over a semiconductor substrate assembly such as a semiconductor wafer. As manufacturing techniques improve and transistor density increases as feature size decreases, one manufacturing step which can create difficulties is photolithography.
A typical photolithography step using positive resist includes the formation of an opening such as a contact to a wafer substrate. To form a contact, a positive photoresist layer is formed over the substrate assembly and the photoresist is exposed in the area where the contact opening is to be formed. The exposed photoresist is removed to expose various underlying layers, then the underlying layers are removed to expose the wafer substrate to which contact is to be made. The opening is typically formed between two structures such as between two adjacent transistor gates. As device density increases the distance between the gates decreases to allow for more transistors per unit area. This distance can decrease only to the limit allowed by photolithography technology, for example allowing for misalignment of the mask or reticle. Photolithography technology is further limited with regard to the minimum size of an opening it can create in the photoresist. Conventional single-layer lithography is capable of resolving line widths less than 0.5 microns on planar, nonreflective substrates. However, when a conventional single layer lithographic technique is used over reflective topography, thickness deviations in the resist lead to poor line width control, and reflections from topographic sidewalls can cause notching. To defeat these problems, dry develop techniques using multiple layers and/or top surface imaging methods have been developed.
Multilayer lithography techniques include a tri-layer patterning scheme. In this process a thick planarizing layer of novolac resin or some other carbon-based polymer is spun onto the substrate assembly and then baked. Next, an intermediate layer which is resistant to attack in an oxygen plasma (spin-on glass, polysiloxanes, aluminum, silicon nitride, and silicon dioxide, for example) is deposited onto the planarizing layer. A thin layer of resist is applied on top of the intermediate layer to function as an initial imaging layer. The imaging layer is used as a mask to transfer the pattern onto the intermediate layer. Once the pattern is transferred onto the intermediate layer, the intermediate layer is used as the mask for transferring the pattern onto the planarizing layer by way of an oxygen-based plasma. A goal of this process is to form vertical sidewalls such that the opening in the resist is the same at the top as at the bottom.
A process which extends the useful life of current photolithography equipment and allows for forming smaller, more accurate contacts to an underlying layer would be desirable.